This invention relates to a process for recovering cyanide from feed material containing cyanide and copper and, more particularly, to such a process for recovering cyanide from solutions or pulps containing copper by ion exchange employing a cyanide eluant.
As is known, the recovery of gold from copper/gold ores containing high levels of cyanide-soluble copper has been difficult in conventional gold carbon-in-pulp (CIP) processes. Many common copper minerals are soluble in the dilute cyanide solutions typical of leach conditions found in gold cyanidation processes. During leaching, certain copper minerals react with free cyanide to form various cuprous cyanide species that compete with the aurocyanide species for available active sites on the carbon. In conventional CIP plants, high levels of free cyanide are maintained to favour the adsorption of Au(CN)−2 anion over the Cu(CN)32− and Cu(CN)43− anions (M. R. Davis et al., Proposed Solvent Extraction Route for the Treatment of Copper Cyanide Solutions Produced in Leaching of Gold Ores, Melbourne, Australia, RMIT University, July 1998, National Symposium on Cost Effective Gold Recovery from Refractory Ores).
Following gold adsorption, the free and complexed cyanide species are generally discarded to a tailings impoundment. Once in the tailings dam, ultra-violet light decomposes free cyanide, naturally detoxifying the tailings solution with time. As the free cyanide level in solution decreases, however, the Cu(CN)32− and Cu(CN)43− species also decompose, liberating additional cyanide. This equilibrium effectively buffers the free cyanide concentration in solution, and can result in significant environmental problems. In addition, trace levels of Cu(CN)−2 in the dam return water consume free cyanide when reintroduced to the cyanide leach circuit.
Detoxification is employed when cyanide-bearing tailings cannot be discarded directly into the tailings impoundment due to environmental or safety-related concerns. The high costs of cyanide destruction and excessive cyanide consumption when treating high cyanide soluble copper ores represent a significant portion of total operating costs, often preventing the development of such projects. This has prompted interest in alternative detoxification technologies that remove copper and recover cyanide for re-use in the leaching process.
Of the several technologies currently available, only one, the so-called Cyanisorb process (trademark of Coeur d' Alene Mines Corporation) has been commercialized to date. In the Cyanisorb process, reliance is on the simplest aspects of cyanide and metal cyanide chemistry and the process does not employ any ion exchange or adsorption technology. In general, the existing technologies are either too expensive to construct and operate, do not recover all of the complexed cyanide or produce a low-value copper by-product that cannot be marketed.
Goldblatt (Recovery of Cyanide from Waste Cyanide Solutions by Ion Exchange, Vol. 51, No. 3, March 1959, Industrial and Engineering Chemistry) teaches the use of ion exchange resins in the recovery of cyanide, water and complexed base materials from the effluent of gold works, but does not teach the recovery of copper. Fleming et al. [U.S. Pat. No. 5,807,421, dated Sep. 15, 1998], teaches a hydrometallurgical extraction process for treating copper feed materials which comprises the steps of:                (a) treating the feed materials with an aqueous cyanide solution having a molar ratio of CN:Cu of >3 thereby leaching copper and gold;        (b) optionally removing and recovering substantially all gold, if present, from the pregnant leach solution or pulp;        (c) contacting the resulting solution or pulp with an anion exchange resin thereby adsorbing copper cyanide at a CN:Cu ratio of <3:1 and gold cyanide, if present, onto the resin;        (d) separating the resin having copper cyanide and gold cyanide, if present, adsorbed thereon from the now depleted solution or pulp containing no copper, gold or cyanide;        (e) treating the loaded resin obtained in step (d) with an eluant containing copper cyanide at a CN:Cu ratio of between 3.5:1 and 4:1 and a copper concentration of at least 10 grams/liter thereby partially eluting copper cyanide and gold cyanide, if present, from the resin and producing an eluate having a CN:Cu ratio of less than 4:1; and        (f) electrowinning copper metal and gold metal, if present, from the eluate produced in step (e).        
There remains a need for improved and simpler processes employing anion exchange solvents or resins to concentrate solutions of cyanide prior to reuse, to allow for the economic recovery of copper in a versatile manner and to use cyanide ion (CN−) as the eluant to produce eluates having low CN:Cu ratios of less than 3.5:1 moles of CN per mole of Cu.